Download Gases and Vapors

CHAPTER 6
Gases and Vapors
Learning Objectives
• Describe the various properties of the chemicals that are
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most important to the practice of industrial hygiene.
Calculate the concentration of a chemical in the air when
provided with the chemical’s basic parameters.
Describe the basic behaviors of gases and vapors in
scientific and mathematic terms.
Identify various methods of collecting and analyzing gas
and vapor concentrations in the air.
Identify the handheld and portable detectors used for
direct measurement of gases and vapors in the
environment.
Normal Temperature
and Pressure (NTP)
• Used for gas and vapor calculations
• 25ºC and 760 mmHg
• At NTP, 1 mole of any gas will occupy 24.45 L.
Concentration of Gases
and Vapors in Air
• ppm = parts of contaminant per million parts of air
• For a gas or vapor of a certain mass at NTP, the
concentration in air can be calculated using:
mg
3 )(24.45 L)
m
ppm =
(molecular weight)
(
• Concentration calculated by volume comparisons:
ppm =
Vcontaminant
× 106
Vair
Vapor Pressure
• The amount of pressure that a liquid exerts on the inside
of a closed container above the surface of the liquid
• At equal temperatures, chemicals with higher vapor
pressures tend to evaporate more quickly than chemicals
with lower vapor pressures.
• In general, chemicals will evaporate more quickly at
higher temperatures.
Vapor Density
• The measure of how heavy the vapor is in air
• Chemicals with higher vapor densities tend to settle near
the floor.
• Chemicals with lower vapor densities tend to rise to the
ceiling.
• Vapors with high densities can displace oxygen from the
workers’ breathing zones, which can lead to
physical/simple asphyxiation.
Specific Gravity (SG)
• The mass of a substance compared to the mass of an
equal volume of water
• Water has an SG of 1.0 gram per milliliter or centimeter
cubed.
Flash Point
• The lowest temperature at which a material gives off
enough vapors to form an ignitable mixture.
Fire Point
• The temperature at which the ignitable mixture will
continue to burn.
Auto-Ignition Temperature
• The point at which a material will ignite without an
external source of ignition.
Brownian Motion
• A measure of how much a gas or vapor molecule moves
in the air.
• The continuous ricocheting of molecules bouncing off one
another which allows them to spread out in space.
Diffusion
• The ability of gases and vapors tend to spread out and
move from locations of high density to those of low
density.
• This movement causes
them to become equally
distributed in a space.
Ideal Gas Law
• When the amount of the chemical is known and it is
entirely evaporated, use the following equation to
calculate the volume that the chemical will occupy at a
given temperature and pressure:
PV = nRT
Generalized Gas Law
• Can be used to quickly determine the pressure, volume,
or temperature of a system:
P1 V1 P2 V2
=
T1
T2
Charles’ Law
• If the pressure in two different conditions remains the
same and only the volume and temperature change, then
the generalized gas law can be reduced to Charles’ Law
with the following equation:
V1 P2
=
T1 T2
Boyle’s Law
• If the volume and pressure in two different conditions
change, but the temperature remains the same, then
Boyle’s Law can be used to calculate the new values:
P1 V1 = P2 V2
Four Reasons to Collect Air Samples
• To obtain an accurate representation of air concentrations
• To identify leaks or releases
• To evaluate the effectiveness of controls
• To assess exposures during particular work processes or
activities
Limit of Detection (LOD)
• The point at which the measurement of an agent first
becomes possible
Limit of Quantitation (LOQ)
• The concentration at which quantitative results can be
measured with a high degree of confidence
Sampling and Analytical Error (SAE)
SAE =
(pump error)2 +(lab error)2
Accuracy and Precision
Determining Sampling Duration
Grab Sampling
Source: SKC Inc.
Detector Tubes, Colormetric
Indicator Tubes, and Pumps
Source: SKC Inc.
Real-time Detection Instruments
• Combustible-gas/multiple-gas monitors
• Thermochemical detectors
• Electrochemical detectors
Real-time Detection Instruments (Cont.)
• Coulometric detectors
• Ionization detectors
Schematic of a flame ionization detector. (A) GC
column exit; (B) detector oven; (C) hydrogen fuel
enters; (D) oxidant enters; (E) positive bias
voltage; (F) flame; (G) collector plates; (H) signal
transmitter; (J) exhaust port.
Source: SKC Inc.
Spectrochemical Direct-Reading
Instruments
Spectrochemical Instruments